A CMUT (capacitive micromachined ultrasound transducer) is a type of MEMS (Micro-Electro-Mechanical Systems) device. The CMUT utilizes electrical forces for the transducer effect. Construction is usually done by manufacturing the CMUT on a silicon substrate. A cavity is formed in the region of the substrate and a thin layer suspended across the cavity to form a membrane. Electrodes are placed at the membrane and at the substrate to form an electrode pair.
Vibration of the membrane can be triggered by applying pressure (for example using ultrasound) or can be induced electrically. Electrical connection to the CMUT, often by means of an ASIC (application specific integrated circuit) facilitates both transmission and reception modes of the device. In reception mode, changes in the membrane position cause changes in electrical capacitance, which can be registered electronically. In transmission mode, applying an electrical signal casses vibration of the membrane.
CMUT devices generally operate with a biasing voltage applied. The CMUT can be operated in so called collapsed mode where the biasing voltage applied is increased above the collapse voltage to restrict the membrane and confine it against the substrate. The frequency of operation of the CMUT device is characterised by the material and physical properties of the membrane, such as the stiffness, and the size of the cavity. The bias voltage and application of the CMUT device also influence the operation mode.
A CMUT is often used in devices for ultrasound imaging applications and in other applications where the CMUT is used to detect fluid or air pressures. A pressure causes a deflection of the membrane that is electronically sensed as a change of capacitance. A pressure reading can then be derived.
A problem with CMUT devices is that a temperature change may also be the root cause of membrane deflection. In reception mode, this gives an inaccurate reading of information derived from the CMUT device. In particular, such a problem can be very limiting for CMUT based pressure sensors used in medical devices. In transmission mode, this causes a reduced efficiency of the electrical to presure power (such as ultrasound waves) conversion